The general architecture of the radio tagged object geolocation systems described in the above-referenced '287 and '046 Patents is diagrammatically shown in FIG. 1 as comprising a plurality of tag emission readers 10 that are installed at precisely geographically known and relatively unobtrusive locations in and/or around the perimeter of an asset management environment 12. The asset management environment contains a plurality of objects/assets 14, to which radio-containing ‘tags’ 16 are affixed. As a result of radio emissions from the tags 16, the locations of the objects 14 can be monitored on a continuous basis by the readers 10 and reported to an asset management data base 20. This data base is accessible by way of a computer workstation or personal computer 26.
In order that the system may locate and track the objects, each radio tag 16 (a circuitry implementation of which is schematically shown in FIG. 2) is operative to repeatedly transmit or ‘blink’ a short duration, wideband (spread spectrum) pulse of RF energy, that is encoded with the identification of its associated object and other information stored in a tag memory. These short duration tag emissions are detected by the tag emission readers 10.
Each tag reader 10 is coupled to an associated reader output processor of an RF processing system 24, which correlates the spread spectrum signals received from a tag with a set of spread spectrum reference signal patterns, in order to determine which spread spectrum signals received by the reader is a first-to-arrive spread spectrum signal burst transmitted from a tag. The first-to-arrive signals are coupled to an object geolocation processor, which performs time-of-arrival differentiation of the detected first-to-arrive transmissions, and locates (within a prescribed spatial resolution, e.g., on the order of ten feet) the tagged object of interest.
The circuitry of a radio tag 16 is schematically illustrated in FIG. 2 as comprising an RF transmitter 40, that includes a relatively coarse oscillator 41, whose output is fed to a first ‘slow’ pseudo random pulse generator 42 and to a strobe pulse generator 44. The strobe generator 44 comprises a time out circuit 46 and a delay circuit 48, the output of which is a low energy receiver enable pulse having a prescribed duration (e.g., one-second wide). This enable pulse is used to controllably strobe a receiver 50, such as a crystal video detector, that is used to detect query signals sourced from a relatively ‘short range’ (e.g., on the order of ten to fifteen feet) low power interrogation unit (such as a hand held wand). Such a low power interrogation unit may be used to more precisely pinpoint an object, for example as an industrial part that may be surrounded by a ‘sea’ of similar parts.
To detect query signals from the interrogating unit, the receiver 50 has its input coupled to a receive port 52 of a transmit/receive switch 54, a bidirectional RF port 56 of which is coupled to an antenna 60. The transmit/receive switch 54 has a transmit port 62 coupled to the output of an RF power amplifier 64, which is powered up only during ‘blink’ mode of operation of the tag.
The output of the ‘slow’ pseudo random pulse generator 42 is a series of relatively low repetition rate, randomly occurring ‘blink’ pulses that are coupled to a high speed PN spreading sequence generator 73 via an OR gate 75. The occurrences of these blink pulses define when the tag will randomly transmit bursts of wideband (spread spectrum) RF energy to be detected by the tag emission readers 10. When enabled by a ‘blink’ pulse, the high speed PN spreading sequence generator 73 generates a spreading sequence of PN chips.
The PN spreading sequence generator 73 is driven at the RF frequency output of a crystal oscillator 82, which provides a reference frequency for a phase locked loop (PLL) 84, establishing a prescribed RF output frequency (for example a frequency of 2.4 GHz, to comply with FCC licensing rules). The RF output frequency produced by PLL 84 is coupled to a first input 91 of a mixer 93, the output 94 of which is coupled to the RF power amplifier 64. Mixer 93 has a second input 95 coupled to the output 101 of a spreading sequence modulation exclusive-OR gate 103. A first input 105 of exclusive-OR gate 101 is coupled to receive the PN spreading chip sequence generated by the PN generator 73.
A second input 107 of OR gate 101 is coupled to receive the respective bits of data stored in a tag data storage memory 110, which are clocked out by the PN spreading sequence generator 73. The tag memory 110 may store parameter data provided by an associated sensor 108 and supplied by a data select logic circuit 109. The data select logic circuit 109 is further coupled to receive data transmitted to the tag from a short range interrogating unit, as decoded by a command and data decoder 112, coupled to the output of the crystal video receiver 50.
A ‘wake-up’ comparator 114 compares the tag address of a query transmission from an interrogation wand with the tag's identification code stored in memory 110. If the two codes match, the comparator causes data in the query message to be decoded by the command and data decoder 112, and written into memory 110 via data select logic circuit 109. The comparator 114 is further coupled through OR gate 75 to the enable input of the PN generator 73, so that, in response to a query message to the tag, its transmitter 40 will generate a response RF burst, in the same manner as it randomly and repeatedly ‘blinks’ a PN spreading sequence transmission containing its identification code and any parameter data stored in memory 110, as described above.
Now, although the tag radios employed in the geolocation system described in the '287 and '046 patents contain circuitry capable of detecting a low energy query signal from a relatively close, low powered interrogator unit, they are not configured to detect (FCC-compliant) communication signals sourced from a relatively remote location, such as, but not limited to, any of the tag emission readers distributed within the infrastructure of the geolocation system.
The ability of a tag to receive a remote communication signal (which implies the use of a higher energy signal) from any location in the geolocation system is very desirable, as it would impart substantial versatility and enhanced functionality to the system. For example, it would allow a tag emission reader to validate reception of a specific tag transmission. (Advantageously, the tag-to-infrastructure communications reliability of the geolocation system of the '287 and '640 patents is inherently very high, since the readers are arranged to ensure that a transmission from any tag will always be received by at least three and preferably four readers.) Communicating remotely to any tag would also allow the system's supervisory computer to initiate a transmission containing information for changing a stored tag parameter (such as its blink rate), or performing an auxiliary function, such as activating a visual or audible indicator installed on the tag.
To be compliant with the extremely limited FCC energy constraints for unlicensed communications (e.g., FCC regulation 15.247), the increased energy required for successfully performing non short range communications mandates the use of some form of spread spectrum modulation. This, in turn, implies the need for what is typically a substantially complex and prohibitively expensive addition to the tag's receiver circuitry, since spread spectrum receivers must be synchronized to the incoming signal to a very high degree of accuracy.